78 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



absolute and relative preponderance of the terrestrial forms in number 

 of species. 13 



The same barrier may separate a whole group of forms from their 

 relatives. The two faunae then have a great similarity of composition, 

 without being exactly identical. The corresponding species of a genus 

 or the subspecies of a species represent each other in the correspond- 

 ing environments of the two areas, without occurring anywhere to- 

 gether. They "vicariate." Such vicariation takes place on a large 

 scale between the mammals of Eurasia and North America where 

 the European and American beaver, the red deer and wapiti, the 

 European elk and the American moose, the reindeer and caribou, the 

 wisent and the American bison, and the European and Canada lynx, 

 replace each other. The bird faunae of the north and south islands 

 of New Zealand exhibit the same relation. The mammals of Kili- 

 manjaro and Kenia exhibit a similar vicariation; monkeys, lemurs, 

 shrews, squirrels, various mice, hyraces, elephants, river hogs, and 

 antelopes are represented on the two mountains by corresponding- 

 species or subspecies. 14 Such vicariation is demonstrable in eighteen 

 pairs of forms, among which are: 



Kenia Kilimanjaro 



Monkeys: Colobus abyssinicus kikuyensis C.a.caudatus 



Lemurs: Galago kikuyensis G. panganiensis 



Squirrels: Heliosciurus keniae H.undulatus 



Hyraces: Procavia crawshayi P.valida 



Adaptive radiation. — When the barrier is an old one and the 

 isolation has been long continued, mutational changes accumulate, and 

 animals of the same origin become so different that they live in wholly 

 different environments. Thus the different habitats come to be filled 

 by related animals, and the whole fauna in such an isolated region 

 exhibits an intimate phylogenetic relationship that is wanting in areas 

 which have been open to continued dispersal. In the isolated area, the 

 different habitats are filled with animals of a single stock, but these 

 are so transformed by this adaptive radiation that they resemble the 

 forms of diverse groups which fill corresponding niches in the environ- 

 ment elsewhere. The Australian region affords an excellent example. 

 Except for the monotremes, a number of rodents, and the dingo, the 

 mammals are all marsupials, and these occupy the most diverse 

 habitats. The wombat takes the place of the marmots; the Tasmanian 

 wolf, that of the wolves; the dasyures, the civet cats; flying phalangers 

 replace the flying squirrels, and the kangaroos fill the place of the 

 larger herbivores. The resemblance is not only one of habits and in 



